Vehicular steering systems include electric power steering apparatus and steer-by-wire systems which are devoid of mechanical linkage.
An electric power steering apparatus is an auxiliary apparatus in which, when while driving a vehicle a driver turns a steering wheel, a motor is operated to provide a supplementary steering force or a steering assist force. In an electric power steering apparatus, a steering torque signal outputted from a steering torque detecting part for detecting a steering torque arising in a steering shaft when the driver turns the steering wheel, and a vehicle speed signal outputted from a vehicle speed detecting part for detecting the speed of the vehicle, are used to drive-control an assisting motor for outputting an auxiliary steering force on the basis of control operation of a motor control part, to reduce the steering force that must be applied by the driver. The motor control part sets a target current value for a motor current passing through the motor on the basis of the steering torque signal and the vehicle speed signal, obtains the difference between a signal pertaining to this target current value (a target current signal) and a motor current signal fed back from a motor current detecting part for detecting the motor current actually flowing through the motor, and performs proportional/integral compensation processing (PI control) on this difference signal to produce a signal for drive-controlling the motor.
Generally, electric power steering apparatus have been developed mainly for use in small vehicles; however, particularly in recent years, from the points of view of fuel economy and expansion of the scope of vehicle control, the need has arisen for larger vehicles (such as passenger vehicles from the 2000 cc class upward) to be provided with electric power steering. In applying an electric power steering apparatus to a large vehicle, because the weight of the vehicle is large, with a construction using a single motor, a large motor outputting a large assisting force is required. Consequently, the size of the motor is large, the freedom of layout with which it can be mounted to the vehicle (its mountability) deteriorates, furthermore a large, special non-standard motor and motor control/driving part therefor become necessary, and the cost of manufacture increases.
To overcome this, as constructions suited to electric power steering apparatus for large vehicles like this, constructions which use two assisting motors have been proposed (see, e.g., International Publication No. WO99/29557, JP-A-2001-260908, JP-A-2001-151125).
However, when an electric power steering apparatus is constructed using two motors, the following problems arise.
When two motors are used, a motor control/driving part is provided for each motor. When two motor control/driving parts are provided in a control unit (ECU), there is a risk of some variation arising between these motor control/driving parts. When such variation arises, a difference arises in the respective voltages applied to the two motors. As a result, variation arises in the motor characteristics, and differences arise in the assisting thrusts produced by the two motors and in their operating speeds.
However, by providing a common gear mechanism for the output shafts of the two motors and connecting them to a rack shaft by this gear mechanism, the rack shaft can be actuated with these differences between the two motors being absorbed. At this time, the difference between the produced torques and speeds of the two motors are balanced by the output of the motor with the higher output being reduced.
In this case, when the degree of the variation between the two motors of the combination is large, the consequent fall in the motor output is large, and the problem arises of the motor output being insufficient even when a preset motor control signal is applied to the motors from the control unit. In particular, when during vehicle travel the steering wheel starts to be turned or is returning to its center position, deterioration of steering feel and deterioration of controllability caused by motor output insufficiency is unavoidable. Because many large vehicles requiring large motor outputs are high-price vehicles, it is important that deficiency of steering feel caused by variation between the two motors in the electric power steering apparatus is eliminated.
Also, there is a risk of it happening that in the neutral vicinity, where the direction of rotation of the motors changes, and when the steering wheel is returning to its central position, due to variations of the motor control/driving parts the rotation of one of the motors momentarily becomes opposite, and the assisting thrusts of the two motors momentarily become opposed and cancel each other out, and responsiveness deteriorates.
Accordingly, in steering apparatus such as electric power steering apparatus having two motors, a motor driving method for a steering apparatus which obtains a balance in the operation of the two motors and of the control/driving part for each motor and which improves steering feel and raises controllability and also provides good responsiveness at all times has been awaited.
Also, when two assisting motors are provided in an electric power steering apparatus as described above, even when one of the motors fails, by means of the other motor it is possible to reduce the manual steering force required from the driver to below what it would be if there were no assisting force from a motor whatsoever. However, the size of the auxiliary steering force from the normal motor, which has not failed, is the still same as it is when the two motors are both operating normally, and to perform the same steering as when the two motors are operating normally the manual steering force load on the driver is larger. When the vehicle is stationary and the steering load is large, with one motor the burden on the driver may be great. In particular, in a heavy vehicle, because the thrust of the steering gearbox is large, the burden on the driver is large.
Reference is now made to the chart of FIG. 22 showing a relationship between the loads on the motors and the driver in a conventional electric power steering apparatus.
In an electric power steering apparatus, the ratio of the auxiliary steering force provided by the motor and the steering force applied by the driver is about 10:1. In particular, in the case of an electric power steering apparatus having two motors, the ratio of the auxiliary steering forces provided by the two motors to the steering force applied by the driver is about 5:5:1.
When the two motors, motor A and motor B, are operating normally, the burden on the driver is small, as shown by the hatching in the graph on the left side of the figure.
If motor A fails, because the auxiliary steering force becomes only that provided by motor B, the burden on the driver increases by the amount of the auxiliary steering force normally provided by motor A. That is, the load ratio of motor A to motor B to the driver becomes 0:5:6. Consequently, as shown by the case of motor A failure in the graph on the right side of the figure, the burden on the driver includes the auxiliary steering force that should be provided by motor A. This is because the size of the auxiliary steering force provided by motor B is the same as it is when motor A and motor B are both operating normally.
With respect to the normal motor, because a limit value of the current to be passed through the motor is set in accordance with the characteristics and durability and so on of the motor, and a current greater than this current limit value cannot be passed through the motor, a larger auxiliary steering force cannot be provided, and the additional load must be taken on by the driver.
When, in an electric power steering apparatus having a plurality of motors, at least one of the motors has failed like this, it is desirable for the amount of auxiliary steering force allocated to the normal motor to be increased, to reduce the burden on the driver.
Also, when an electric power steering apparatus is constructed using two motors, the kinds of problem described below arise; however, first, problems of electric power steering apparatus having a single motor will be considered.
Normally, in an electric power steering apparatus having one motor, the output shaft of the motor is connected to the steering system by way of a gear mechanism, which is a power transmission mechanism. This gear mechanism may have any of various forms. A typical one is the pinion assist type electric power steering apparatus, which has a speed reducer provided on a pinion gear shaft and a motor with its output shaft connected to this speed reducer. With a pinion assist type electric power steering apparatus, in a rack-and-pinion gearbox made up of a rack shaft having a rack gear and a pinion gear driving this provided on a steering wheel shaft, a motor and a speed reducer are provided serving the pinion gear. The pinion gear is driven by the motor via the speed reducer. By this means, steering force assistance of the steering system corresponding to the steering force is carried out.
In this electric power steering apparatus having a single motor, with the gear mechanism made up of a pinion gear and a rack gear, if the torque ripple of the motor is large, this torque ripple passes through the gear mechanism and manifests as vibration in the steering system and spoils the steering feel, and also the operating noise of the motor increases. Consequently, the product quality of a vehicle equipped with this electric power steering apparatus decreases.
In the case of a pinion assist electric power steering apparatus having two motors as steering system assisting motors, because each of the two motors is connected to the steering system by way of the rack-and-pinion mechanism described above and each of the two motors imparts a vibration caused by motor torque ripple to the steering system through this gear mechanism, the above-mentioned impairment of steering feel and motor operation noise become still more marked. This is a problem which arises generally in steering apparatus having two motors. Also, the problem arises with both brushless motors and motors with brushes.
Consequently, in steering apparatus such as electric power steering apparatus having two motors on a steering system, there has been a need to suppress motor torque fluctuation caused by torque ripple arising in each motor and thereby to reduce vibration in the steering system, improve steering feel, and raise controllability.